A known typical voice coil actuator is comprised of a core, an axially polarized cylindrical magnet, a base plate, a rod, and two pole pieces. The rod and the magnet are coaxially mounted on the base plate, such that the rod extends through the center of the magnet. The two pole pieces are coaxially mounted respective to the magnet and the rod. The first pole piece has an inner diameter less than the inner diameter of the cylindrical magnet and the second pole piece has an inner diameter greater than the diameter of the rod. Therefore, a gap remains between the facing sides of the pole pieces. A coil is coaxially suspended within the gap.
In co-pending application Ser. No. 07/925,085, also assigned to the assignee of the present application, and incorporated by reference herein, a voice coil actuator utilizing radially oriented magnets is disclosed. The voice coil actuator includes a magnetic flux conductive material core, a magnet and an electrical current conductive coil. The core has a first surface and a continuous channel disposed in the first surface. The channel has a pair of opposing walls. The magnet is disposed in intimate contact with a first wall and spaced from an opposing wall so that a gap remains between the magnet and the opposing wall. The magnet has a first face of a first magnetic polarity facing the first wall and a second face of a second, opposite magnetic polarity facing the gap. The magnet is further spaced from a bottom of the channel so that magnetic flux is substantially normal from the second face across the gap to the opposing wall. The electrical current conductive coil is disposed moveably in the gap such that an electrical current in the coil develops a magnetic force on the coil in a direction substantially normal to the magnetic flux to displace the coil in response to the magnetic force.
The use of radially oriented magnets in the voice coil actuator minimizes flux leakage from the core, and thereby maximizes efficiency. It is desirable, however, to also reduce the radially oriented magnet structures weight or mass, while retaining the high level of magnetic flux density. Alternatively, it may be desirable to increase the level of magnetic flux density without increasing the weight and mass of the radially oriented magnet structure.
Therefore, a need exists for a radially oriented magnet structure that provides a high level of magnetic flux density without requiring increased weight or mass of the magnet structure.